Light Detection And Ranging (LiDAR) systems are attractive for use in many applications, such as driverless automobiles, farm equipment, and the like. Laser range finding is used to determine the range from a source to an object by sending a pulse of light in the direction of the object, detecting a reflection of the pulse, and determining the time required for the light to travel to and return from the object (i.e., its time-of-flight).
A typical prior-art LiDAR system creates a local map around a vehicle by performing laser range finding in several directions and elevations around the vehicle. Prior-art systems accomplish this in different ways, such as using an array of laser sources, rotating a single laser source about an axis through the vehicle, or directing the output signal from a single source about the vehicle using a rotating mirror or prism, or a stationary reflective cone. For example, US Patent Publication No. 20110216304 describes a LiDAR system based on a vertically oriented array of emitter/detector pairs that are rotated about an axis to provide a 360° horizontal field-of-view (FOV) and vertical FOV of several tens of degrees. This system emits multiple pulses at a high repetition rate while the emitter/detector assembly is scanned about the vehicle. The resultant distance measurements form the basis for a three-dimensional simulated image of the scene around the vehicle.
The requirements for a LiDAR system used in automotive applications are quite challenging. For instance, the system needs to have a large FOV in both the horizontal and vertical directions, where the FOV is supported over a distance that ranges from approximately 5 meters (m) to approximately 300 m. Further, the system must have high resolution, as well as an ability to accommodate a changing environment surrounding a vehicle that could be travelling at relatively high speed. As a result, the system needs to be able to update the simulated environment around the vehicle at a high rate. In addition, an automotive LiDAR system needs to be able to operate at both day and night. As a result, the system needs to accommodate a wide range of ambient light conditions.
The need for high-resolution performance would normally dictate the use of high laser power to ensure sufficient return signal from objects as far away as 300 m. Unfortunately, eye safety considerations limit the laser power that can be used in a LiDAR system. The safety threshold for the human eye is a function of wavelength, with longer wavelengths being, in general, safer. For example, the Maximum Permissible Energy (MPE) for a nanosecond-range light pulse is approximately six orders of magnitude higher at 1550 nanometers (nm) than at 980 nm. Unfortunately, the solar background is quite high at 1550 nm and can degrade measurement sensitivity in this wavelength regime. As a result, many prior-art LiDAR systems operate in the wavelength range from approximately 800 nm to approximately 1050 nm at low optical power levels, thereby sacrificing system performance.
Some prior-art LiDAR systems do operate 1550 nm; however, FOV is normally restricted to mitigate the effects of the solar background radiation. Unfortunately, such an operational mode reduces the update rate for the system. To develop a sufficiently large image of the surrounding region, therefore, multiple systems having overlapping fields of view are required, which increases overall cost and system complexity.
For these reasons, a low-cost, high-performance LiDAR system suitable for vehicular applications would represent a significant advance in the state-of-the-art.